Just a few years ago, the runway where Trump and other world leaders arrived was heavily contaminated with a highly toxic chemical the United States had used during the Vietnam War.

Now, thanks to a years-long transnational cleanup effort advanced by Sen. Patrick Leahy, D-Vt., the soil around the airport is on track to be decontaminated within a few months.

“I’m not sure they would have let them land there if we hadn’t done that,” Leahy said in an interview last week.

For decades after the Vietnam War, the lingering impact of the United States’ use of Agent Orange was an obstacle in efforts to normalize relations between the two countries.

As leaders gathered halfway around the world for the summit, Leahy and others involved with mitigating the legacy of the chemical reflected on how far things have come — and what still remains to be done.

During the war, the United States sprayed the herbicide over swaths of the landscape, a tactic to beat back the dense jungle to better track Viet Cong positions. The defoliant contained dioxin, a highly toxic chemical. It is linked to a number of severe ailments and birth defects.

A study found that Agent Orange contamination was particularly strong around a few former U.S. military bases,including Danang, where the chemical was stored and mixed during the war.

“This probably affected our relationship as much as anything because it was a daily reminder of the war,” Leahy said. “And then it became a generational reminder.”

Leahy recalled visiting a family during one trip to Vietnam whose children had severe disabilities related to Agent Orange. Though the children were born long after the war ended, they lived in an area where the soil was contaminated with the chemical.

Charles Bailey, the co-author of a forthcoming book on the legacy of Agent Orange, said that when the two countries first started relations during the mid-1990s, they weren’t addressing the issue. Though Vietnamese officials wanted to discuss it, Americans did not.

The health impact of Agent Orange on American veterans of the Vietnam War has been a controversial subject and an issue the Department of Veterans Affairscontinues to grapple with.

Bailey headed work by the Ford Foundation in Vietnam, which was key in efforts to pinpoint dioxin hot spots. He credited Leahy with leading the push for the United States to help heal the legacy of Agent Orange in Vietnam.

Leahy had already been involved with efforts to address another remnant of the war. During George H.W. Bush’s presidency, the senator had worked to provide funding for people affected by land mines.

Leahy, a longtime member of the powerful Appropriations Committee, began an effort about a decade ago to find money to collaborate with the Vietnamese government on a dioxin cleanup of the area around the Danang airport. He traveled to Danang in 2014 when the project launched.

The cleanup involved building a concrete-walled structure roughly the size of a football field, according to Leahy. Contaminated soil is then put into the container and heated to at least 635 degrees — hot enough to sterilize the soil, according to a 2016 Washington Postarticleon the project.

The effort will cost the U.S. more than $100 million by its conclusion.

Bailey noted the environmental remediation funding was accompanied by funding to help support families affected by dioxin.

The effort has been important for the United States’ relationship with the Southeast Asian country, he said.

“It sends a huge message to Vietnam and everyone else in the world that we do clean up our messes,” Bailey said.

The project attracted attention from world leaders and the international press with last week’s APEC summit. In a joint statement released Sunday, Trump and Vietnamese President Tran Dai Quang recognized the completion of the cleanup at Danang as part of ongoing cooperation to address the impacts of the war.

As the Danang project comes to a conclusion, sights are now turning to Bien Hoa — another former U.S. military base, near Ho Chi Minh City, where dioxin contamination is far more widespread. Leahy said the cost is estimated to be between three and five times that of the first project.

Leahy, who voted to end the war when he was in his first term in the Senate, believes the United States has a “moral responsibility” to help Vietnam address the legacy of dioxin. The cost of the initiative has been split between the State Department and Pentagon.

“This is a tiny, tiny, tiny fraction of the billions we spent on that war,” he said.

Though it may be challenging to find funds to assist Vietnam in addressing the Bien Hoa site, Leahy expects he will find support from his colleagues in Congress.

“Some will join me because it’s the humanitarian thing to do,” he said. “Others will join me because it improves the security of the United States.”

Elizabeth Hewitt is the criminal justice reporter for VTDigger. She grew up in central Vermont and holds a graduate degree in magazine journalism from New York University.